Rain sensor, in particular for a motor vehicle

ABSTRACT

A rain sensor for a motor vehicle is proposed, having a transmitter ( 16 ) emitting radiation, a receiver ( 18 ), and a holographically embodied diffractive element ( 22   a,    22   b ) that has a linear-grating-like structure.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a rain sensor according to the species defined in the independent claim.

[0002] Numerous rain sensors are known, for example from DE 197 01 258, in which the radiation of a transmitter is coupled into the window of a motor vehicle by a holographically embodied diffractive element, totally reflected at the outer side of the window, and directed or focused onto a receiver by a further diffractive element. In such sensors, the diffractive elements are embodied as volume phase holograms that are introduced into films made of photopolymers.

[0003] Such films are complex to produce, expensive to manufacture, and problematic in terms of their environmental properties.

ADVANTAGES OF THE INVENTION

[0004] The rain sensor according to the present invention having the features of the main claim has the advantage that because of the line-like structure of the diffractive element, it is more economical to manufacture and a wider spectrum of materials can be used. In addition, the environmental properties, for example, are thereby improved. Larger sensitive areas can moreover be provided, thereby improving the response characteristics of the sensor unit.

[0005] The actions set forth in the dependent claims yield advantageous developments of and improvements to the features recited in the main claim.

[0006] It is particularly advantageous if the element diffracts the radiation into a first and into a minus-first order, since in this fashion two sensitive areas can be achieved with only one transmitter and receiver, with no need to increase the radiation intensity. Additional costs can be saved in this fashion, and the interference resistance further reduced.

[0007] A particularly good signal-to-noise ratio of the receiver signal is achieved by the fact that the element exhibits focusing properties.

[0008] A particularly compact sensor is advantageously obtained if a further element is provided that is partially embodied as a retroreflector. In this fashion the receiver can be positioned in the vicinity of the transmitter, or the transmitter can be used simultaneously as a receiver.

[0009] If at least one element and/or one further element is embodied as a multi-layer structure, more-complex beam paths can also be represented in simple fashion.

[0010] It is particularly advantageous in this context if one element exhibits several structures that are superimposed in one layer.

[0011] Advantageously, the elements or the further elements are positioned on the window of the motor vehicle. It is particularly advantageous in this context if the diffraction efficiency of an element lies between 20 and 80%, in particular between 40 and 80%, preferably between 60 and 80%.

[0012] Particularly good signals can be represented by the fact that the diffraction efficiency of the element is less than 100%, in particular less than 90, preferably less than 85%.

[0013] In a particularly economical embodiment of the rain sensor, the latter has more receivers than transmitters, since the transmitters are more cost-intensive than the receivers.

[0014] In addition, multiple receivers make possible a more intelligent signal evaluation and thus an improved adaptation of wiper speed to the rain intensity.

DRAWINGS

[0015] A number of exemplifying embodiments of the invention are depicted in the drawings and described in more detail in the description below. In the drawings:

[0016]FIG. 1 shows a rain sensor according to the present invention in a schematic cross-sectional depiction;

[0017]FIG. 2 shows a variation of a sensor according to the present invention from FIG. 1;

[0018]FIG. 3 shows a beam path through a diffractive element, in a schematic cross-sectional depiction;

[0019]FIG. 4 shows a variation of the beam path of FIG. 3;

[0020]FIG. 5 shows a beam path such as the one in FIG. 4, but with a multi-layer diffractive element;

[0021]FIG. 6 shows one possible positioning of the diffractive elements of a sensor according to FIG. 2; and

[0022]FIG. 7 shows a further positioning of the diffractive elements in a variation of FIG. 6.

DESCRIPTION OF THE EXEMPLIFYING EMBODIMENTS

[0023]FIG. 1 shows a rain sensor 10 according to the present invention in a schematic cross-sectional depiction. It is substantially made up of a housing 12 in which a circuit board 14 is positioned. Circuit board 14 carries a transmitter 16 and a receiver 18 which are embodied as semiconductor components using SMD technology. Housing 12 sits on a window 20, for example the windshield of a motor vehicle, and is attached thereon. This can be accomplished, for example, by adhesive bonding; but retaining elements such as grooves or lugs, which are positioned on or in window 20 and coact with attachment elements of housing 12, are also possible.

[0024] Diffractive elements 22 a, 22 b, which are made up of a film equipped with a holographic linear grating, are adhesively bonded onto window 20. One of diffractive elements 22 a is positioned in the region on window 20 in which the radiation of transmitter 16 directly strikes window 20. Film 22 a is embodied in such a way that it couples the radiation of transmitter 16 into window 20, specifically in such a way that the radiation is totally reflected at the clean and dry glass-air interface facing away from rain sensor 10.

[0025] Second diffractive element 22 b is positioned at the point on window 20 at which the radiation of transmitter 16 emerges from window 20 after total reflection. The linear grating is selected in such a way that the totally reflected radiation is deflected, in particular is focused, toward receiver 18.

[0026]FIG. 2 depicts a further exemplifying embodiment of rain sensor 10 shown in FIG. 1. Here one transmitter 16 and two receivers 18 are positioned on circuit board 14. In the region of transmitter 16, diffractive element 22 a having a linear grating is positioned on window 20. The linear grating is once again embodied in such a way that the radiation of transmitter 16 is directed into the first and minus-first diffraction orders. Corresponding thereto, further diffractive elements 22 b are positioned in the region of receiver 18 in such a way that the light from the first and minus-first diffraction orders is directed onto receiver 18. Cross-shaped measurement paths can also be implemented by suitable selection of element 22 a (FIG. 7).

[0027]FIG. 3 depicts transmitter 16 and diffractive element 22 a of FIG. 2 in detail. Diffractive element 22 a is embodied as a hologram film having a linear grating, in such a way that the radiation proceeding from transmitter 16 perpendicularly onto the hologram film is diffracted into first order 24 and minus-first order 26. This diffraction effect at the grating is commonly known in physics, and described in numerous textbooks. The first and minus-first diffraction orders have the same intensity. The zero order, into which no intensity also may flow if applicable, is located in the transmission direction i.e. in the straight-line direction viewed from transmitter 16.

[0028] On the receiver side, diffractive element 22 b is configured in such a way that only first-order light is directed to receivers 18, and the other orders, i.e. the minus-first order or higher orders, are blocked out. This is made possible, for example, by the use of blazed gratings or so-called holographic lenses.

[0029] Diffractive elements 22 a, 22 b can moreover possess particular focusing properties, for example in order to convert the divergent, wedge-shaped beam of transmitter 16 into one having a cylindrical cross section. In diffractive elements 22 a of this kind, minus-first order 26 is in some circumstances divergent, and can also be used as useful radiation by using suitable additional actions.

[0030]FIGS. 4 and 5 depict a variation of FIG. 3. Diffractive element 22 a here is made up of a superimposition of diffractive structures, each rotated 180 degrees with respect to one another, each of which diffracts predominantly into the first order. In FIG. 4, both structures are superimposed in one layer. This can be achieved, for example, by double exposure of a hologram. As shown in FIG. 5, however, these two structures can also be produced in separate layers and then, for example, laminated onto one another. This results in two first-order sub-beams, diffracted in opposite directions and focusing in a desired fashion. The minus-first-order rays that are also present are not utilized here. FIG. 6 shows an assemblage of diffractive elements 22 a, 22 b. The path of the radiation of transmitter 16 is indicated schematically by the arrows. Center diffractive element 22 a is positioned in the region of transmitter 16 (not shown here), and diffracts the radiation in the first order, via the totally reflecting glass-air interface (not depicted), to external receivers 18.

[0031]FIG. 7 depicts a variation of the assemblage shown in FIG. 6. Five diffractive elements 22 are shown here. Four of these five diffractive elements 22 a, 22 b are positioned in the shape of a square in whose center fifth diffractive element 22 a is positioned. The transmitter (not depicted) is positioned in the region of fifth diffractive element 22 a and directs radiation into the four outer diffractive elements 22 b. Outer diffractive elements 22 b positioned in the region of receiver 18 can of course also be positioned in the shape of a rectangle, in the shape of a rhombus, but also in the shape of a triangle or another geometric shape. The number of linear structures superimposed in central diffractive element 22 a of transmitter 16 is, however, equal to the number of receivers 18 that are present.

[0032] In principle, elements 22 b can also be embodied here as retroreflectors, and can radiate the radiation back into the region of the transmitter, where receiver 18 is then positioned. In addition, elements 22 b can also direct the radiation toward a shared receiver 18, so that, for example, two elements 22 b positioned next to one another guide the radiation onto one shared receiver 18. 

What is claimed is:
 1. A rain sensor, in particular for a motor vehicle, comprising at least one transmitter (16) emitting radiation, at least one receiver (18) receiving radiation of the transmitter (16), and at least one holographically embodied diffractive element (22 a, 22 b), wherein the element (22 a, 22 b) has a linear-grating-like structure.
 2. The rain sensor as defined in claim 1, wherein the element (22 a, 22 b) diffracts the radiation into a first and into a minus-first order.
 3. The rain sensor as defined in one of the foregoing claims, wherein the element (22 a, 22 b) exhibits focusing properties.
 4. The rain sensor as defined in one of the foregoing claims, wherein at least one further element (22 a, 22 b) is provided that is at least partially embodied as a retroreflector.
 5. The rain sensor as defined in one of the foregoing claims, wherein at least one element (22 a, 22 b) and/or one further element (22 a, 22 b) has a multi-layer structure.
 6. The rain sensor as defined in one of the foregoing claims, wherein at least one element (22 a, 22 b) and/or one further element (22 a, 22 b) exhibits several structures that are superimposed in one layer.
 7. The rain sensor as defined in one of the foregoing claims, wherein at least one element (22 a, 22 b) and/or at least one further element (22 a, 22 b) is positioned in and/or on a window of the motor vehicle.
 8. The rain sensor as defined in one of the foregoing claims, wherein an element (22 a, 22 b) and/or a further element (22 a, 22 b) exhibits a diffraction efficiency of 20 to 80 percent, in particular 40 to 80 percent, preferably 60 to 80 percent.
 9. The rain sensor as defined in claim 8, wherein the diffraction efficiency of the element (22 a, 22 b) and/or of the further element is less than 100 percent, in particular less than 90 percent, preferably less than 85 percent.
 10. The rain sensor as defined in one of the foregoing claims, wherein more receivers (18) than transmitters (16) are provided. 